In the metallurgical industry, there are a number of processes in which a product has to be temporarily cooled down, stored and possibly transported, and later remelted. Such a product is defined herein as a "product for remelting" (PFR).
The most common PFRs are the ferroalloys like ferro-chromium, ferro-manganese, ferro-nickel and ferro-silicon, which are used as a source of alloying elements during the manufacture of certain types of steels. The furnaces that produce these PFRs are often geographically distant from the site of their end use. There are also some other metals like aluminium, copper and zinc, and some of their alloys, that are similarly produced in a different place from where they are used. These materials therefore need to be converted from the liquid form to some type of solid form that can be handled and transported.
Another type of PFR arises and is later consumed in the same plant. This typically occurs when a downstream production unit is taken off line for maintenance, but the upstream unit continues to produce. The hot metal that continues to come from the upstream unit cannot be held molten in storage until the downstream unit comes back on line, and consequently must be converted into a solid form that can later be remelted or blended in. The PFR is then effectively a buffer between stages. An example of a plant where this could occur is an integrated iron and steel works, where a blast furnace produces pig iron that is then fed to a steel plant for conversion to steel, that goes on in turn to a continuous caster. In this case, if the steel plant stops the pig iron must be taken elsewhere, while if the continuous caster stops the steel must be handled in some other way.
Existing methods of handling PFRs are mainly the following.
Bed Casting, and Pooling
Here, the molten material is poured into moulds on the ground, and after cooling is broken up into lumps of the required size. A problem here is the unavoidable production of a certain amount of unwanted fines.
Ingot Casting, Including Casting Strands and "Chocolate Moulds"
In this process, the liquid material is poured into moulds. These may be either individual moulds, or may be assembled in a continuous loop as a casting strand. It is a relatively expensive process, tends to be labour intensive, and requires careful operation.
Granulating
In essence, this involves breaking up a stream of molten material either by means of a water jet or on a target, with the material then falling into a tank of water. The particles produced tend to be smaller than desired by end users, and the product is usually wet when it comes from the process, but the product is suitable for easy mechanical handling.
There are of course many other methods for casting hot materials, but these are of rather marginal relevance to PFRs. One such process is atomising, in which the molten material is converted to a fine powder by means of a high pressure jet of water or gas. This powdered product is too fine for remelting, and is typically used for powder metallurgical processing, for welding electrodes or as a heavy-medium for mineral separation.
Existing Types of Granulation
In one version of this process, a strong jet of water at a speed of between 5 and 15 m/s is directed to collide with a falling stream of material. This breaks up the material into droplets between about 1 and 20 mm in size which fall into a bath of water and solidify. In another implementation, a stream of molten material is broken up by a refractory target placed in its path, and the resulting droplets, varying up to about 25 mm in size, then fall into a bath of water. The former process is widely known in the industry as the Showa Denko process, and the latter as the Granshot process. Another process, which is generally used in the granulation of slag, has a near-vertical stream of molten material colliding with strong horizontal jets of water, with the mixture being swept along a near-horizontal launder filled with rapidly-flowing water. Lastly, lead shot is made by allowing droplets of molten material to fall about 45 meters through air in a device known as a shotting tower. The resulting droplets, which are usually a millimeter or two in diameter, solidify as they fall through the air.
The techniques used in the aforementioned processes have by now entered the public domain, --see for example the Granshot process patented in 1975 in U.S. Pat. No. 3,888,956. However, there are some new variations that have been patented more recently. For example, South African patent ZA 90/4005A, describes a scheme like an extension to the Granshot process, in which the refractory element on which the molten metal stream impacts is oscillated vertically. Other patents, ZA 91/2653 and U.S. Pat. No. 5,258,053 (1993), describe a process in which molten metal is run onto a refractory target shaped like a launder and then into a tank of water. The outlet of this target is close to the surface of the water, and the water within the tank is kept reasonably still, with a gentle and uniform flow of less than 0.1 m/s being directed at right angles to the submerged metal stream.
U.S. Pat. No. 4,192,673 addresses the problem of particles, of ferro-nickel in their specific case, that form flat wrinkled shapes during granulation, because of the generation of carbon monoxide (CO) gas as the ferro-alloy cools. The inventors claim that this can be prevented by the addition of deoxidising agents such as particularly aluminium, but also ferro-silicon, ferro-manganese and the like.
An example of a newer development for the granulation of slag is disclosed in U.S. Pat. No. 4,374,645. Here, the molten slag is first contacted with a high-speed jet of warmer water to break it up, after which it falls into a slower cooler stream of water.
Deficiencies of the Prior Art
The following are some of the main deficiencies.
The bed-casting and mould-casting processes require labour to be present in the vicinity of the casting operation. Molten metal, particularly in the quantities employed in iron, steel and ferro-alloy production, is exceedingly dangerous.
The exposure of hot metal to the air often generates fumes. Large pools of hot metal therefore tend to be associated with rather more pollution than is desirable.
As mentioned previously, the process of breaking up a block of cast alloy generates a portion of fines which have a lesser commercial value. The granulation process lessens the problem of fines, but the dimensions of the granules produced by the existing processes remain somewhat smaller than those that the end users consider optimum.
The granulation process can sometimes produce "corn flakes", which are light fluffy paper-like particles, instead of normal granules. These may subsequently break up into smaller particles, which then create similar problems to the fines from casting.
Existing granulation processes are susceptible to occasional explosions, often associated with an accumulation of a large mass of hot metal under the water.
Granulated material is normally wet when it comes from the granulator. This wetness can give problems when the material is used subsequently and such material must usually be dried.
Identification of the Need
Most users would seem to prefer lumps of ferro-alloy in about the 20 to 100 mm size range. This is said to be because lumps of that size range will fall rapidly through the slag layers covering a typical bath of molten metal. It is also a requirement that the material should feed easily through the existing materials handing systems. The material should also be dry. The existing granulated materials feed easily, but the particles tend to be too small. Lumps of broken-up cast ferro-alloy would seem to be able to meet the size requirement, but there is then an unavoidable loss in the form of fines. Some users would also seem to have a preference for the form of granules over that of broken-up material. There appears to be no prior art which can produce, without significant disadvantages, a granular material with the form and size preferred by the users.
There is therefore, notwithstanding the efforts of others, still a specific need for a reliable, safe, convenient and cheap process to convert molten metal by direct solidification, without intervening crushing, into solid pieces as lumps of a size and form that are acceptable to the end users. These lumps should preferably be substantially globular or biscuit shaped, where the longest dimension is typically between 20 and 100 mm. Besides the requirements mentioned, these lumps should ideally be capable of withstanding the rigours of storage, transport and handling without degrading to fines. The technique to produce these lumps should not be more hazardous nor require more human labour and maintenance than the presently-used methods. It is obviously a requirement of such a process that it should not introduce excessive quantities of undesirable impurities into the ferro-alloy. The process should also be simple to construct and operate, particularly relative to existing methods.